1. Field of the Invention
The present invention relates to gas bearings and particularly to gas bearings for use in moveable object tables or other components of lithographic projection apparatuses.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the "lens"; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems and field lenses, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation and such elements may also be referred to below, collectively or singularly, as a "lens". Any refractive, reflective or catadioptric elements in the radiation or illumination systems may be based on a substrate of glass or another suitable material, and may be provided with either single- or multi-layer coatings as desired. In addition, the first and second object stages may be referred to as the "mask stage" and the "substrate stage", respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such "multiple stage" devices the additional tables may be used in parallel, or preparatory steps may be carried out on more stages while one or more other stages are being used for exposures. Twin stages lithographic apparatuses are described in International Patent Application WO 98/28665 and WO 98/40791.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosentive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus--which is commonly referred to as a step-and-scan apparatus--each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the "scanning" direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally M&lt;1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Lithography apparatuses for the production of Intergrated Circuits (ICs) are being designed to cope with ever larger wafer and/or reticle sizes, which leads to increased size and mass for the substrate and/or mask stage. At the same time there is a continual demand for increased placement accuracy and stability.
The increase in object stage size and mass results in increased servo-bandwidth requirements in the apparatuses. This increases the desired natural resonant frequencies of mechanical components under control. As a result, designers of lithographic apparatus desire to manufacture components in the object stages out of materials that are lighter, more rigid, and/or have higher resonance frequencies. Ceramic materials, such as aluminum oxide, aluminum nitride, silicon carbide or silicon nitride, are considered promising but their poor machinability in the sintered state is a problem.
Gas bearings ("air bearings") are commonly used in positioning in a lithography apparatus because of there cleanroom compatibility and frictionless running. Use of air bearings entails providing air restriction features such as minute apertures or surface grooves on one side of the bearings surfaces, which in turn are connected to air supply channels. In a bearing made of metal, this is easily achieved; however in a ceramic block it is considerably more difficult, because the restriction features are to be much smaller in dimension than the air supply channels in the rest of the block.